The present invention relates to a multi-slice X-ray CT apparatus that is capable of scanning helically (hereinafter referred to as a multi-slice CT); and, more particularly, the invention relates to a multi-slice CT having a correction processing function for performing correction processing of measured spiral projection data.
Nowadays, a R/R type (the third generation) CT system is mainly being used. In such a system, an X-ray source and detectors, which are arrayed in a circle so as to point to a focal spot of said x-ray source, are positioned so as to face a space into which the object to be examined is inserted. The x-rays from the x-ray source are collimated to form fan-shaped x-ray beam, and they are irradiated to the imaging plane of the object to be examined. Data acquisition is performed by measuring the attenuated x-rays transmitted through the object to be examined. The procedure for acquisition of data is performed at intervals 0.1xcx9c0.5 degree. For example 600xcx9c1200 items of projection data are acquired during one revolution.
A detector is comprised of many detector elements, and the output of each detector element is corrected as a digital data value using measurement circuits to compose data(view) of a number of elements at each angle. This view data is successively transferred from a rotation system to a stationary system with respect to the transmitting path. After pre-processing, such as characteristic correction of detector elements, and log conversion of the transferred data to an image processing device in a stationary system, a slice image is reconstructed with a known algorithm, such as a filtered back projection method.
As an one applied example of such an X-ray CT system, a spiral CT (helical CT) apparatus is known which makes it possible to achieve a faster examination when measuring with a rotating x-ray source and detector, together with a moving table for supporting the object to be examined. In such a spiral CT apparatus for scanning spirally around the object to be examined, it is necessary to acquire the projection data at a slice position using interpolation from data acquired spirally for acquiring a specified slice image. Such an interpolation method is disclosed for example in U.S. Pat. No. 4,789,929. By performing such interpolation processing, the presence of a motion artifact can be reduced.
Furthermore, there is a multi-slice CT apparatus that makes it possible to measure a plural number of projection data simultaneously by dividing detectors into a plural number of rows. In such a multi-slice CT, the view is simultaneously corrected for the number of rows. Thus, in case of conventional stationary table scanning, a plural number of slices can be simultaneously imaged. When spiral scanning is performed in such a multi-slice CT, it is necessary to reconstruct the data by performing interpolation processes the same as a single slice, or weighting corresponding to it. U.S. Pat. No. 5,541970 discloses a method which achieves helical correction by composing a weighting coefficient to interpolate with the nearest complementary beam. In addition in Japanese Patent Laid-Open No. 9-285460, a method to increase the continuity by use of a smoothing weighting coefficient in a Z axis direction is proposed. However in such traditional multi-slice CT spiral scanning, it is impossible to cope with the changing of the relationship between the row number of the detectors and the helical pitch, and there is no expansibility, such as increasing the order of interpolation.
Thus, an object of the present invention is to provide a multi-slice CT apparatus that makes it possible to obtain a high quality slice image and that is capable of coping with the changing of the spiral pitch when spiral scanning is performed in a multi-slice CT operation.
Another object of the present invention is to provide a multi-slice CT apparatus with which it is possible to reduce the x-ray dose applied to a patient during a multi-slice CT operation.
To achieve the above-mentioned objects, a multi-slice CT apparatus according to the present invention has multi-element detectors arranged in a plural number of row in the axial direction, and a moving patient table for supporting the object to be examined for movement in the axial direction. In the apparatus, x-rays transmitted through the object to be examined by a rotating x-ray source are detected by said detectors, and a plural number of spiral projection data are acquired. The apparatus further has correction processing means for performing correction processing of measured spiral projection data, and image reconstruction means for acquiring a slice image reconstructed with corrected projection data.
The correction processing means generates a plural number of different multi-slice spiral weighting to cope with the spiral pitch amount of table movement during one rotation relative to a row interval of said detectors. One of said plural number of multi-slice spiral weighting is selected in accordance with the spiral pitch during measuring to apply spiral projection data for each row, and spiral projection data at each row is combined after weighting is applied.
One embodiment of a multi-slice CT system according to the present invention involves correction processing means which operates to change the weighting region of spiral projection data of a processing target in accordance with the spiral pitch during measurement. Thus, data in the neighborhood of a measuring position can be used as a data for interpolation, and a high quality image can be obtained.
Another embodiment of a multi-slice CT system according to the present invention involves correction processing means which operates to generate a multi-slice spiral weighting coefficient applied to spiral projection data at each row and to combine spiral projection data at each row after said weighting is applied. Such means for generating said multi-slice spiral weighting coefficient sets a virtual detector that is at a different position from the actual detector, and sets multi-slice spiral weighing about all rows of projection data comprising said actual detector and virtual detector.
A virtual detector can be set to make all row number comprising said actual detector and virtual detector be P, when the actual detector row number N is smaller than the spiral pitch P(P greater than N), and a virtual detector can be positioned between actual detector in the neighborhood. Weighting for applying projection data of said virtual detector can be shared with weighting of projection data at the actual detectors in the neighborhood. On the other hand, a virtual detector can be positioned outside of the measuring region in the axial direction of an actual detector row, and complementary data can be used as projection data of said virtual detector, whereby weighting for said complementary data is shared with projection data of the actual detectors in the neighborhood.
By using the concept of a virtual detector, interpolation for using appropriated weighting is made possible so as to produce an image of high quality, even in a case where the spiral pitch is larger than the detector row number. And, when one slice projection data is acquired with interpolation during multi-slice helical scanning, a discontinuity due to changing of a pair of real measured data for interpolating in a slice is canceled so as to produce an image of high quality.
Furthermore, as another embodiment of a multi-slice CT system according to the present invention, the x-ray source comprises means for controlling the rows of said detectors corresponding to the spiral pitch of the table movement per one rotation relative to a detector row interval. The number of rows can be set appropriately in accordance with the spiral pitch, and the x-ray dose applied to a patient can be reduced by controlling if the row number needs a lot.